What they say may seem clear, and yet not reflect what they really believe.
I give blood every two months. As I recall the questionnaire, you can do almost anything imaginable (sex with prostitutes, accidental needle stick, receiving blood transfusion, etc.) and get a one year deferral. But if you are a man who had oral sex with a man one time twenty years ago, that's a permanent "deferral." No science behind this.
A history of male-to-male sex is associated with an
increased risk for exposure to and transmission of certain
infectious diseases, including HIV, the virus that causes
AIDS. Men who have had sex with other men represent
approximately 2% of the US population, yet are the
population
most severely affected by HIV. In 2010, MSM accounted for at
least 61% of all new HIV infections in the U.S. and an
estimated 77% of diagnosed HIV infections among males were
attributed to male-to-male sexual contact.
Looks like science, namely statistics and medicine. If 61% of new HIV infections are coming from MSM then it's higher chances than then other criteria you've listed combined (i.e. non-MSM new HIV infections are only 39%, which is much less than 61%).
Exactly what conclusions do you draw from those stats? Yes, I remember from my mathematics degree that 39<61, but what does that have to do with anything? What groups do you think you are comparing?
(Especially) If a man has not had sex with another man for a year, and has tested clean during that time, the stats you cite do not support excluding him from donation. Again, this is considering that blood transfusions, sex with prostitutes (!!), and accidental needle sticks only earn a twelve-month deferral.
Do you see a stat that says a gay man is more likely to be HIV+ than an active prostitute? Because that seems to be a relevant comparison.
I donate blood to save lives. No blood transfusion is risk-free, but there is also a risk/cost to excluding healthy willing donors of clean blood.
FDA is pretty clear on why they don't want blood donations from men who had sex with other men.